🚀 go-pugleaf

I have a situation where if variable x contains the string "XXX" I want
one thing to happen; if it contains anything else (say, "YYY ZZZ") I want
something else to happen.  How do I perform this test?

The fragment

.se x = 'XXX'
.if &x = XXX;.th FOO;.el BAR
.if XXX = &x;.th FOO;.el BAR

gives  FOO BAR  when processed by SCRIPT; I should think it would give
FOO FOO .  If I replace the first statement with

.se x = 'YYY ZZZ'

then I get the diagnostic statement
INVALID KEYWORD FOR CONDITION ON .IF.
'TRY' LINE 3: .if YYY ZZZ = XXX;.th FOO;.el BAR
(which I understand), and still  FOO BAR .

Questions:
(1) Why aren't the two .if statements in either of the two examples
entirely equivalent?
(1a) Why didn't I get the same diagnostic for line 2 of the second
example?
(2) What is the syntactically correct way to test the contents of a
variable whose value may be a string with embedded spaces?  (I've tried
messing around with single quotes, double quotes, both, etc., and also
RTFM, all to no effect.)  Any insight gratefully accepted.
                                             /b